Investigators are looking into whether gunman Stephen Paddock scoped out bigger music festivals before shooting scores of country music fans in Las Vegas.

Paddock booked rooms overlooking the Lollapalooza festival in Chicago in August and the Life Is Beautiful show near the Vegas Strip in late September, according to authorities reconstructing his movements before he undertook the deadliest mass shooting in modern US history.

It was not clear if he aborted plans to carry out massacres at those sites.

Investigators looking into Paddock also came across mention of Fenway Park, Boston police spokesman Mike McCarthy said, though he provided no further details.

The details came to light as investigators struggled to figure out why the high-stakes gambler opened fire on a crowd of 22,000 on Sunday night from the 32nd floor of the Mandalay Bay hotel casino in Las Vegas. He killed 58 people and injured nearly 500 before taking his own life.

A US official said authorities are looking into the possibility Paddock planned additional attacks, including a car bomb.

Authorities previously disclosed Paddock had 1,600 rounds of ammunition in his car, along with fertiliser that can be used to make explosives and 50 pounds of Tannerite, a substance used in explosive rifle targets.

Investigators have been combing through his background but remain stumped as to his motive.

The profile developed so far is of a "disturbed and dangerous" man who acquired an arsenal over decades, Sheriff Joseph Lombardo said. But investigators have been frustrated to find that he lived a "secret life," Mr Lombardo said, "much of which will never be fully understood".

The weekend before the massacre, he rented a room through Airbnb at the 21-storey Ogden condominiums in downtown Las Vegas and stayed there during a music festival below that included Chance the Rapper, Muse, Lorde and Blink-182.

"Reasons that ran through Paddock's mind is unknown, but it was directly at the same time as Life Is Beautiful," the sheriff said.

Police were reviewing video shot at the high-rise to check Paddock's movements. His renting the condo was curious because as a high-roller, he could have easily got a free room at one of the casino hotels on the Vegas Strip.

In early August, Paddock booked a room at Chicago's 21-storey Blackstone Hotel that overlooked the park where the Lollapalooza alternative music festival was being held, though there is no evidence he actually stayed there, a law enforcement official said.

The hotel confirmed a Stephen Paddock made a reservation but said he never checked in.

Lollapalooza draws hundreds of thousands of music fans every year to Grant Park.

Although Paddock killed himself as a Swat team closed in, the sheriff said it appeared he had planned to survive and had an escape plan.